Sorafenib is the first systemic tlierapy witii proven efficacy against liepatoceilular carcinoma (HCC). However, since HCCs evolve rapidly to circumvent sorafenib's effects, survival in HCC patients is prolonged only about 2 months. Although the anti-tumor effect of sorafenib was initially attributed to inhibition of RAF/IVIEK/ERK and vascular endothelial growth factor (VEGF) receptor pathways, recent clinical trials have challenged this hypothesis by showing inconsistent results with even more potent and/or selective MEK and VEGF inhibitors. The limited efficacy of these agents emphasizes the need to understand how HCCs evade sorafenib treatment. We show that HCC cells that survive sorafenib treatment harbor activated l\/!EK and ERK, and that ERK activation mediates their viability after treatment. In Aim 1 we will investigate the role of MEK/ERK activation-consistently seen in all the HCC cell lines we have tested-as a cell autonomous mechanism of evasion from sorafenib. The evasion from VEGF blockade also emphasizes the need to test whether sorafenib induces changes in tumor stroma (e.g., hypoxia) that facilitate HCC growth after treatment. We found that hypoxia levels, stromal- derived factor 1a (SDFIa) expression, Gr-1+ bone marrow-derived cells (BMDCs) number and fibrosis are all increased after sorafenib treatment in HCC. We will examine tumor stromal mechanisms of evasion from sorafenib treatment in HCC, and dissect the causal links between SDFIa upregulation, BMDC recruitment and tumor fibrosis in HCC in Aim 2. Finally, our preliminary data suggest that adding MEK/ERK or SDF1a/CXCR4 inhibitors to sorafenib significantly delays HCC growth compared to sorafenib alone. In Aim 3, we will evaluate tumor response and toxicity after combining pharmacologic inhibitors of MEK or CXCR4 with sorafenib in (syngeneic and spontaneous) orthotopic HCC models in immunocompetent mice. The goals of this project will be achieved in close collaboration with the multidisciplinary PPG team.